lib-sieve: include extension: forgot to check variable identifier syntax.

src/lib-sieve/plugins/include/cmd-global.c

src/lib-sieve/plugins/include/ext-include-variables.c

src/lib-sieve/plugins/variables/ext-variables-arguments.c

src/lib-sieve/plugins/variables/ext-variables-name.c

src/lib-sieve/plugins/variables/sieve-ext-variables.h

tests/extensions/include/rfc-ex2-default.sieve